Field of the Invention
Transmission methods in which portable data carriers exchange information with a terminal in a contactless fashion, i.e. by inductive or capacitive coupling or generally over a wire-free electromagnetic transmission link, are becoming increasingly widespread. Here, the portable data carrier is normally also supplied with power by the terminal in a contactless fashion, for which purpose a high-frequency carrier signal is used on which, in addition, the data are modulated, on the one hand, and which, on the other hand, serves directly as a clock signal in the data carrier.
For the transmission of data, in most applications the carrier signal is merely switched on and off so that a 100% amplitude shift keying (ASK) modulation takes place. The carrier signal is then usually not gated out or switched off or left unchanged during the entire bit time window, defined by an "electrical" bit period and referred to below as a time period, in order to transmit a logic "1" or "0" but rather only during a significantly shorter time. The gating out interval is for example a quarter or a third of the bit time window or time slot in length. As a result, between two states that lead to gating out, the carrier signal is present for long enough to ensure the supply of power and of the clock.
A possible coding is provided by the Miller code. Here, a logic "1" is transmitted by a gating out interval in the center of a time slot. A logic "0" is transmitted via a gating out interval at the start of a time slot, a logic "0" which is transmitted after a logic "1", not being represented by a gating out interval in the time slot, in order to avoid excessively small distances between gating out intervals.
FIG. 6A represents a signal in the Miller code with which the amplitude of a carrier signal is to be modulated. It represents a bit sequence 101000110. The time slots are indicated by vertical dotted lines. A gating out interval has approximately a quarter of the duration of a time slot. FIG. 6B represents the associated frequency spectrum which is determined computationally, the signal from FIG. 6A having been modulated onto a carrier signal with a frequency of 10 MHz. As is clear, an envelope of the side bands with a (sinx/x) shape is produced, it being possible to determine a distance between the maximum amplitude of the side bands and the carrier of approximately 18.2 dB.
When the worst case of a modulation with a sequence of ones 111111111 is assumed, as illustrated in FIG. 7A, a frequency spectrum is obtained such as is illustrated in FIG. 7B. Here the distance between the side bands and the carrier is still just approximately 10.5 dB.
According to the regulations for radio approval and the interim standard ETS 300330 (ETSI, September 1994) on which these are based, an ISM frequency of 13.56 MHz, for example, is provided for the carrier, with a level limit of 42 dB.mu.A/m @ 10 m. The bandwidth for the carrier is just +/-7 kHz, so that the modulation spectrum with the described modulation and bit coding in accordance with the Miller code is completely outside this. In the aforesaid standard, the level outside is limited with 20 dB.mu.A/m @ 10 m, that is to say 22 dB under the carrier, the measurement bandwidth being 10 kHz. The described system with 100% ASK modulation with Miller coding exceeds this limit, as stated before, and can obtain radio approval only if modulation is not carried out continuously and the mean value is used, something which is acceptable according to previous, clear regulations, but not according to ETS 300330 and the FCC regulation in the USA, which prescribe a quasi-peak evaluation.
The document "Infrarot-Datenubertragung wird schneller" [Infrared Data Transmission is Becoming Faster] by Helmuth Lemme from Elektronik [Electronics] 3/1996, pages 38 to 44, it is known to use a pulse-position modulation in order to increase the data rate when transmitting in the infrared range, but each pulse is assigned two bits. However, here the pulse-position modulation is used to obtain a higher data rate with the same number of transmitted pulses per time unit, so that the amplitude of the side bands of the frequency spectrum does not change, which is however also not significant in this case. However, since the intention is to aim for the maximum possible pulse rate here, there are no intervals between the pulses so that, given a succession of bit sequences 11 and 00, the carrier signal is gated out over the duration of two pulses. With infrared transmission this does not constitute a further problem since here there is an oscillator for generating the clock in the receiver.
In methods such as those on which the present invention is based, in which the carrier signal acts directly as a clock signal, this would however lead to an unacceptably long loss of the clock signal.